Results: In the quest to synthesize a useful material not found in nature, a
scientific team developed a multidimensional analysis approach, leading to
the first direct measurement of ordering in the material at the atomic scale.
Through investigations of the double perovskite La2MnNiO6,
or LMNO, the team, led by Pacific Northwest National Laboratory scientists, showed
that combining multiscale synthesis, characterization, and modeling techniques could
lead to a predictive understanding of complex materials systems. In turn, this understanding
could help scientists precisely engineer these systems.

The results
are published in Chemistry of Materials.
Lead author Dr.
Steven Spurgeon, a postdoctoral associate at PNNL, his mentor, materials
expert Dr.
Scott Chambers; and colleagues shed light on a fundamental challenge in
materials science: many proposed materials have never been synthesized because
existing characterization and modeling approaches fail to capture the inherent
complexity of solid state systems.

Spurgeon,
Chambers, and colleagues at PNNL, North Carolina State University (NCSU),
Sandia National Laboratories, and instrument
manufacturer Gatan Inc., put forth an approach to connect atomic-scale structure to
macroscale magnetic properties.

Why It Matters: LMNO has been studied by scientists for the
past decade because of its value in thermoelectric and spintronic applications (see sidebar). LMNO, a ferromagnetic
semiconductor material exemplifies the challenges associated with
structure-property engineering of multicomponent material systems. Much debate
has revolved around what chemical structure LMNO forms and how to engineer the
structure to improve its magnetic properties.

Existing materials
characterization techniques have limitations. For example, while it
is straightforward to measure overall magnetic properties, figuring out how
atomic structure gives rise to those properties is much harder.
Traditional analysis relies heavily on X-ray scattering, which can give
detailed but indirect insight into a material's structure. The challenge is to
directly image the material at the atomic scale, connect those images to
larger length scales, and, finally, determine how structure controls
properties. This study has met that challenge, bringing scientists closer to
their goal of controllably engineering such a material.

Methods: Spurgeon's background is in scanning transmission electron
microscopy (STEM), one of the instruments of choice for materials scientists. TEM
has been around since the 1930s, and it can provide amazing images of even
single atoms. But TEM images do not always represent a material's overall
structure.

The PNNL
team developed a new approach to characterize material locally using STEM,
along with a three-dimensional (3D) analysis technique called atom probe tomography
(APT). "This combination gave us beautiful insight into how the material looks across
a range of length scales," said Spurgeon. "By using STEM at the atomic scale,
APT at the midrange (mesoscale), and magnetic characterization at the
macroscale, we could understand how this hierarchy of structure gives rise to
properties. That's huge."

This
approach requires advanced instrumentation unavailable at most labs,
so DOE's Environmental Molecular Sciences Laboratory (EMSL) located at PNNL is
key. "The suite of tools at EMSL allows us to do unique materials science. Only
a handful of labs worldwide have these kinds of resources in house," said
Spurgeon. The study also took advantage of the Advanced Photon Source at
Argonne National Laboratory, another DOE national user facility.

PNNL
scientists have also used molecular beam epitaxy (MBE), for synthesizing materials a
single layer of atoms at a time. With MBE, researchers can use a monitor to watch crystal layers grow, and even count single layers of material as they
form. "The ability to synthesize materials this way takes years to learn, and
it's as much art as science," said Spurgeon.

Once the
PNNL team successfully synthesized LMNO, measurements confirmed the stability
of its structure. But previous theory calculations suggested that its magnetic
properties were not as good as they should have been. "There should have been a
much larger magnetic moment than what we observed," said Spurgeon. "So we threw
every technique we could at it, including using the X-ray beam line at the
Advanced Photon Source, to look at its overall magnetic properties and
structure. Nothing made sense."

He wondered
if something was going on locally that the X-ray analysis was missing. "Our STEM
analysis showed that even though we got the crystal structure we expected, the
manganese (Mn) and nickel (Ni) atoms were swapping places. When Mn and Ni
atoms alternate in chains through the crystal, they give rise to good magnetic
properties, but if these atoms are swapped or out of place, poor magnetic
properties result."

Using STEM,
the PNNL and Sandia scientists found that though the LMNO
crystal structure was correct, Mn and Ni atoms were randomly distributed
throughout it (see left image at top), which is difficult to see using
X-ray measurements because of the similarity of the two atoms.

Heating
the material caused the atoms to reorganize into a much more ordered structure (right
image at top). "The arrangement of Mn and Ni went from a random state to a
checkerboard," said Spurgeon. At the same time, they saw a huge increase in
magnetic properties. By analyzing the material before and after heat treatment,
they confirmed the ordering was greatly enhancing the material's magnetic
properties.

Enter RevSTEM. While the
magnetic properties were now much improved, theory suggested that they could be
even better. The scientists knew they wanted to directly image the material at
larger length scales, moving from a handful of atoms to broader regions, so
they used a new STEM technique called RevSTEM developed by collaborators at NCSU.
Because of the way STEM images are typically acquired, they are very sensitive
to small movements of the sample, making it hard to accurately measure bonds
between atoms. The NCSU group developed a method that uses many fast images to
correct for these distortions.

Spurgeon
explains, "By correcting the distortions, we can obtain more precise measurements of crystal structure and then measure the angle
between atoms and compare it to what we expect. Any deviation in these angles can
affect magnetic properties. That's the tie back." Using RevSTEM, the team could
map the LMNO crystal structure to look for any disorder over larger areas.
They found broad regions where the LMNO was slightly structurally
disordered.

This hint of
structural disorder led the team to a shocking discovery: the supposedly pure
material was actually laced with small, 2- to 5-nanometer regions of nickel
oxide (NiO). 3D atom maps measured using APT showed these
regions were distributed throughout the film and were so small that the X-ray
analysis could easily overlook them. Because NiO is antiferromagnetic, it could
greatly reduce the magnetic properties of the ferromagnetic LMNO. But why was
NiO forming?

The team
turned to PNNL scientist Dr. Peter
Sushko, who specializes in density functional theory (DFT), a popular
technique for calculating crystal properties. His models showed that for the
synthesis conditions used in the study, the material is at a tipping point
where LMNO and NiO can form together.

As Spurgeon
explains, "That's the downside to MBE. We can't put much oxygen into the material.
If you are using low oxygen pressure, there's a specific regime where you can
get phase separation. We hadn't considered that before. But the upside is that
within this growth regime, we could make a material where you might want phase separation, such as in a
nanocomposite structure. To engineer these structures controllably would be of
great value."

What's Next? While LMNO
has been studied for many years, scientists are just now beginning to
understand the direct connection between structure and magnetic properties. More
important, the use of advanced synthesis, characterization, and modeling
approaches will allow them to gain deep insight into how materials form
and what gives rise to their properties.

Acknowledgments

Sponsors: This work was supported by the U.S. Department of Energy Office
of Science, Office of Basic Energy Sciences Division of Materials Sciences and Engineering. Much of the work
was performed in EMSL, a national science user facility sponsored by DOE's
Office of Biological and Environmental Research and located at PNNL. Other
work was performed at Sandia National Laboratories for DOE's National Nuclear
Security Administration, at the Analytical Instrumentation Facility at North
Carolina State University, and the Advanced Photon Source at Argonne National
Laboratory.

Additional Information

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Spintronics in a nutshell

Spintronics
is a new type of memory electronics that couples electricity and magnetism in
the solid state, in contrast to hard drives, which use moving parts to apply an
external magnetic field. The hard drive is slow, energy intensive, and prone to
mechanical failure. Spintronics, on the other hand, consist of a single
material, similar to the kind of flash memory used in a smartphone. With such a
solid-state device, you can apply an electric field to it and store information
magnetically.

In one sentence:
A multidimensional analysis approach developed at PNNL brings synthesis of a valuable material closer.